In internal combustion engines with one or more pistons which move inside respective cylinders, each piston comprises one or more rings, which undergo severe efforts when the engine is working.
A way to ensure the wear resistance of a ring, so that it has a long shelf-life, is to apply a layer of coating over the base metal of which it consists. The coating specifically developed to resist wearing and abrasion, keeps the properties of development of the ring even after millions of cycles of ring displacement inside the cylinder.
A first representative prior art is North-American U.S. Pat. No. 6,698,763, which refers to a piston ring with a stainless steel base to which a nitrided layer is initially applied, wherein the nitrided layer comprises a diffusion layer and a compound layer, the latter being applied over the first.
After the application, the upper and lower circular corners of the ring are chamfered all over their extension in such a way that, in these places, all the nitrided layer is removed, exposing the base material. Finally, in the outer side surface of the ring, which stays in contact with the cylinder wall, a ceramic layer of coating is applied.
Removing the chamfers prevents stress from building up in these places and significantly reduces the emergence and propagation of cracks.
A second representative prior art is North-American U.S. Pat. No. 6,508,473, which refers to a piston ring with a stainless sell base to which a nitrided layer is applied all over its extension, except to the outer surface in contact with the cylinder, to which another coating is applied, through the process of ion plating or PVD (Physical Vapor Deposition), comprised by chromium nitride (in its phases CrN and/or Cr2N) or titanium nitride. This second coating and the nitrided layer are separated, without overlapping each other or making contact, in respective regions adjacent to the upper and lower surface sides. The distance between both coatings presents values ranging from 0.001 millimeters (mm) to 0.3 mm.
With this embodiment, occasional cracks that emerge have their propagation limited, avoiding the formation of cracks of such an extension that lead to the detachment of the recoating.
A third representative prior art is disclosed in Japanese patent document JP 2002-61746, which refers to a piston ring with a base in steel or molten iron which has a concave recess on the outer surface (the one in contact the cylinder wall). Initially, a nitrided layer is applied to the base; however, this layer is not applied to, or is removed exactly from, the outer surface, due to its properties of little ductility. Over the outer surface, a layer of chromium nitride is applied by the process of ion plating or PVD (Physical Vapor Deposition), being later lapidated. The resulting ring has good abrasion resistance and appropriate resistance to the detachment of recoating.
A fourth representative prior art is disclosed in Japanese patent document JP 5-172248, which refers to a piston ring with a metallic base to which a hard ceramic layer of chromium or titanium nitride is applied. Immediately afterwards, a nitrided layer is formed on the side, lower, upper surfaces and on the inner face of the ring. The outer corner, turned to the cylinder wall, does not receive any nitrided layer and is provided, therefore, only with a hard ceramic layer initially deposited by the process of ion plating or PVD (Physical Vapor Deposition).
However, please note, none of the documents above-mentioned considers that the efforts suffered by the ring vary in a non inconsiderable manner, that is, that the ring does not suffer all the efforts in a completely homogenous manner.
By means of extensive studies, tests and simulations, the applicant discovered that most of the efforts suffered by a piston ring and with potential to cause recoating detachment or structural damages lie in a region substantially opposite to its gap, that is, substantially located at 180° from the gap. In this region, due to the efforts suffered, relevant micro-cracks emerge on a coating layer formed, for instance, by CrN, and they propagate up to the moment the ring breaks. In a set of tests carried out, it was verified that, in the other portions of the ring, the emergence of cracks on the CrN layer remained at acceptable values.
Without relying on the removal of chamfers or other procedures that make the manufacture of the piston ring expensive, the applicant presents a completely new and original product, which has no nitrided layer in the portion facing the gap, brilliantly fulfilling all the requirements of performance and durability.
Purposes of the Invention
Therefore, one of the purposes of this invention is to provide a piston ring, preferably but not mandatorily as a compression ring, conceived for use in internal combustion engines or compressors, having a nitrided layer all over its extension, except on the region which is substantially opposite to its gap or slit.
Another purpose of the present invention is a piston ring, preferably but not mandatorily as a compression ring, conceived for use in internal combustion engines or compressors, which has a coating of chromium nitride deposited through the process of ion plating or PVD (Physical Vapor Deposition) and which has reduced values of formation of potentially destructive micro-cracks.
Finally, another purpose of the present invention is the process for manufacturing the ring aforesaid.